This invention relates in general to electrostatography and, more specifically, to a flexible electrostatographic imaging member having a functionally improved anti-curl backing layer.
Flexible electrostatographic imaging members are well known in the art. Typical electrostatographic flexible imaging members include, for example, photoreceptors for electrographic imaging systems and electroreceptors such as ionographic imaging members for electrographic imaging systems. Generally, these imaging members comprise at least a supporting substrate layer and at least one imaging layer comprising thermoplastic polymeric matrix material. The "imaging layer" as employed herein is defined as the dielectric imaging layer of an electroreceptor or the photoconductive imaging layer of a photoreceptor. In a photoreceptor, the photoconductive imaging layer may comprise only a single photoconductive layer or a plurality of layers such as a combination of a charge generating layer and a charge transport layer.
Although the discussions hereinafter focus on electrophotographic imaging members, the problems encountered therewith are equally applicable to electrographic imaging members.
In the art of electrophotography, an electrophotographic imaging plate comprising at least one photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the electrophotographic imaging plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in electrophotographic imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. Generally, the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer. During an electrophotographic imaging process, the outer surface of the charge transport layer is normally charged in the dark with a uniform negative electrostatic charge and the conductive layer is utilized as a positive electrode. The photoconductive layer is capable of photogenerating holes and injecting the photogenerated holes into the contiguous charge transport layer. The charge transport layer in this embodiment must be capable of supporting the injection of photogenerated holes from the photoconductive layer and transporting the holes through the charge transport layer. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as a negative electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electron/hole pairs and injecting the photogenerated holes into the charge transport layer when the imaging member surface is uniformly charged with a positive charge while the conductive layer beneath serves as a negative electrode to receive the injecting holes. The charge transport layer in this embodiment, again, is capable of supporting the injection of photogenerated holes from the photoconductive layer and transporting the holes through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium--tellurium, selenium--tellurium--arsenic, selenium--arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirely. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles.
When one or more photoconductive layers are applied to a flexible supporting substrate, it has been found that the resulting photoconductive member tends to curl. Curling is undesirable because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, developer applicators and the like during the electrophotographic imaging process thereby adversely affecting the quality of the ultimate developed images. For example, non-uniform charging distances can be manifested as variations in high background deposits during development of electrostatic latent images. To eliminate the imaging member curling problem, a coating solution may be applied to the side of the supporting substrate opposite the photoconductive layer after drying, to form an anti-curl layer after drying which counteracts the tendency to curl and to provide the desired imaging member flatness. Unfortunately, several difficulties have been encountered with this anti-curl layer.
In a typical 6,000 feet long roll of electrophotographic imaging member production webstock, the charge transport layer comes to intimate contact with the anti-curl coating. The high surface contact friction generated between the charge transport layer and the anti-curl coating have been found to cause the formation of dimples, creases, and localized delamination of internal imaging member layers. Moreover, areas of polymer deformation developed in the layers of the imaging member, again due to the high surface contact friction of charge transport layer against the anti-curl layer in the webstock roll, have also been implicated in water mark like copy printout defects as the imaging member webstock is converted into belts and cycled in electrophotographic imaging machines. Since the anti-curl layer is an exposed outermost layer, it has further been found that during cycling of the photoconductive imaging member belts in electrophotographic imaging systems, the relatively rapid wearing away of the anti-curl layer also results in the curling of the photoconductive imaging member. In some tests, the anti-curl layer was completely removed or worn away after 150 thousand to 200 thousand cycles. This anti-curl layer erosion problem is even more pronounced when photoconductive imaging members in the form of belts are supported by a belt support module design which contains stationary guiding surfaces. During dynamic belt cycling, these stationary guiding surfaces, cause the anti-curl layer to wear away very rapidly and produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Since a typical anti-curl layer, using film forming polycarbonate such as Makrolon (available from Bayer AG), has a surface energy of approximately 42 dynes/cm, the anti-curl layer has the tendency to collect toner residues, dirt particles, and debris onto its outer surface and fuse them into comets as well as protrusion spots over the anti-curl layer surface which degrade the imaging belt cycling/motion quality and promote fatigue charge transport layer cracking.
Attempts have been made to overcome the above problems. However, the solution of one problem often leads to the creation of additional problems. For example, although the addition of micro-crystalline silica, at a 10 weight percent level in the anti-curl layer has been found to decrease charge transport layer/anti-curl layer surface contact friction and enhance wear resistance of the anti-curl layer, but excessive welding horn wear is observed when this electrophotographic imaging member belt is fabricated by the use of ultrasonically welding process of overlapping ends of an imaging member sheet. This wear is the result of the horn contacting with the melted anti-curl coating and charge transport layer materials when this molten mass is ejected to form splashing on either side of the overlapped ends.
It has also been observed that when conventional belt photoreceptors, using a bisphenol A polycarbonate (such as Makrolon, available from Bayer AG) anti-curl layer, are extensively cycled in precision electrophotographic imaging machines employed belt supporting backer bars and ROS exposure systems, an audible squeaky sound is generated due to high contact friction interaction between the anti-curl layer and the backer bars. Moreover, undesirable defect print marks are formed on copies as a result of localized cumulative deposition of toner particles, dirt, and debris brought by the anti-curl coating onto the surface of the backer bars. These deposits force the photoreceptor upwardly and interferes with the toner image development process. It is important to note that debris accumulation on the backer bars has also been found to gradually increase the dynamic contact friction of the two interacting surfaces, i.e., anti-curl layer and backer bars. This increase in dynamic contact friction forces a gradual increase in the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases.